One of problems involved in a spread-spectrum communication system of a direct spreading (DS) system is high-level CM jamming or interference which often disables communication. In order to overcome the problem, various filters using a SAW are proposed. Some typical examples are as follows:
(i) CW interference is suppressed by an adapting processing using a programmable transversal filter (C. M. Panasik 1982 Ultrasonics Symp. Proc. pp 100-103) PA1 (ii) A chirp filter performs Fourier transform to suppress interfering signal components by controls through an RF switch or by combination of cutting and removing functions, and a chirp filter performs reverse Fourier transform. A convolver is used for subsequent processings subsequent to the reverse Fourier transform, thereby to simplify the system. (J. Gevargiz et al., 1985 Ultrasonics Symp. Proc. pp 108-113) PA1 (iii) An input signal is spacially distributed depending on frequencies, using an inclined chirp transducer as an input transducer, so as to suppress CW interference, using an elastic nonlinearity in a region where a high level signal component propagates. (M. F. Lewis, ELECTRONICS LETTERS 21, 1985, pp 573-574)
These filters, however, involve the following drawbacks:
In the filter of (i) above, since the impulse response of the transversal filter is controlled by controlling the weighting of respective taps, it substantially behaves as a transversal notch filter. Therefore, it is difficult to obtain a narrow-band notch characteristic, and the filter undesirably suppresses relatively wide frequency components near the CW interference.
In the filter of (ii) above requiring Fourier transform and reverse Fourier transform, its signal processing is complicated.
In the filter of (iii) above, the use of the elastic nonlinearity requires significantly large power for the input.